You should get used to using either Firebug (a plugin for Firefox) and/or Google Chrome's inspector (which is there by default on Chrome). If you're using any of those, you can right click->inspect element, and then hit the 'Network' tab.

Refresh your page while you're in the Network tab, you'll see the HTTP requests. This will help you figure out if there is a mistyped path, or any unreachable files. Additionally, you will be able to see any javascript errors when you click the 'console' button. The console button is at the bottom left, it looks a bit like this: >=